1. Field of the Invention
The present invention relates to an image recording apparatus and an image correction method, and more particularly, to image correction technology for preventing faults in an image caused by abnormalities in recording elements.
2. Description of the Related Art
In recent years, inkjet recording apparatuses have come to be used widely as data output apparatuses for outputting images, documents, or the like. The inkjet recording apparatus drives recording elements, such as nozzles provided in a recording head, in accordance with data so as to form data onto a recording medium, such as recording paper, by means of ink ejected from the nozzles.
In the inkjet recording apparatus, a desired image is formed on a recording medium by causing a recording head having a plurality of nozzles and a recording medium to move relatively to each other while causing ink droplets to be ejected from the nozzles.
If an abnormality, such as a fault or decline in performance, occurs in a recording element provided in a recording head, then a dot abnormality occurs. Namely, a dot is omitted or the desired dot is not formed in the position where the dot was originally to have been formed on the recording medium, and therefore, a fault occurs in the recorded image. When performing one-pass recording of recording elements using a full line recording head comprising a row of recording elements of a length corresponding to the entire printable width of the recording medium, image defects (artifacts), such a stripe-shaped omissions or non-uniformities in density, are formed extending in the relative conveyance direction of the recording head and the recording medium, thus leading to a marked decline in image quality.
In order to prevent decline in image quality of this kind, methods have been proposed in which a recording element producing a fault or decline in performance is discovered, and omitted dots are compensated for by using other recording elements, such as recording elements located peripherally to the discovered recording element.
Japanese Patent Application Publication No. 2003-136763 discloses an image correction method for inkjet recording described, in which same-color correction (correction using an adjacent nozzle) and different-color correction (correction using different colored ink in the same position) are combined when providing correction for nozzles suffering an ejection failure. In a range where the correction data does not exceed the maximum recordable value, same-color correction is performed, and in a range where the correction data exceeds the maximum recordable value, different-color correction is performed, in such a manner that image defects occurring due to nozzles in an ejection failure state or nozzles producing defective ejection can be prevented satisfactorily without lowering the recording speed.
Japanese Patent Application Publication No. 2003-136764 discloses an image correction method for an inkjet recording apparatus in which a prescribed pattern is output with the object of determining head shading and ejection failures. This prescribed pattern is read in, and correction data is created after performing a visability characteristics computation (or an averaging process within a range of 50 to 300 μm) with respect to the read pattern image, in such a manner that image deterioration caused by nozzles of the recording head producing ejection failures is prevented.
Japanese Patent Application Publication No. 2002-234216 discloses a method for reduction of artifacts in reproduced images, in which artifacts are reduced during the reproduction of an image having a plurality of image points, by taking account of the reproducibility of pixels, which is dependent on the image reproduction apparatus (pixel position error, pixel density error).
Japanese Patent Application Publication No. 09-083796 discloses an image processing apparatus, in which a periodic variation equal to or lower than a quantization level is applied to a signal corrected for density non-uniformities, in such a manner that noise equal to or lower than the quantization level can be alleviated. The noise intrinsic to error diffusion can also be prevented.
However, when correcting an image by using the recording elements peripheral to an abnormal recording element, there is a risk that artifacts, such as dot omissions or non-uniformities in density, may actually become more conspicuous depending on the image processing method (dot forming method) used.
In the image correction method for inkjet recording described in Japanese Patent Application Publication No. 2003-136763 and the image correction method for an inkjet recording apparatus described in Japanese Patent Application Publication No. 2003-136764, image correction is performed by using nozzles located adjacent to an ejection failure nozzle or nozzles of a head corresponding to another color; however, there is no specific disclosure with regard to the processing performed with respect to an abnormal nozzle in a half-toning process.
Furthermore, in the method for reduction of artifacts in reproduced images described in Japanese Patent Application Publication No. 2002-234216 and the image processing apparatus described in Japanese Patent Application Publication No. 09-083796, there is no disclosure with regard to the image correction technology used when a recording element is not functioning correctly, for instance, when it is suffering an ejection failure.